The present invention relates to a surface acoustic wave device having natural single-phase unidirectional transducer characteristics.
In recent years, mobile communication terminals such as cellular phones have rapidly become widespread. Such terminals are desired to be reduced in size and weight from the viewpoint of portability. In order to achieve reduction in size and weight of the terminals, electronic parts used therein are required to be reduced in size and weight, too. For that purpose, a surface acoustic wave device, i.e., a surface acoustic wave filter, which is advantageous for reduction in size and weight, is often employed in the high frequency section and the intermediate frequency section of the terminals. The surface acoustic wave device is formed by providing interdigitated electrodes for exciting, receiving, reflecting or propagating a surface acoustic wave on a piezoelectric substrate.
As the surface acoustic wave device, a transversal surface acoustic wave filter, which selectively picks up a signal of a desired frequency, has been practically used widely. Characteristics that are demanded of the filter include a wide pass band, a small size, a low loss and good temperature characteristics. In order to realize a filter having such characteristics, it is important to select an appropriate piezoelectric substrate and an appropriate electrode structure.
Among characteristics demanded of a substrate for a surface acoustic wave device, it is important that the surface wave velocity of the surface acoustic wave (hereinafter sometimes referred to as a SAW velocity) is small for achieving reduction in size; xcexa2, i.e., the square of electromechanical coupling coefficient (hereinafter xcexa2 is also referred to as electromechanical coupling coefficient) is large for achieving a wider pass band; and the temperature coefficient of a center frequency when used for a filter or of a resonance frequency when used for a resonator is small.
Quartz, lithium niobate, lithium tantalate, lithium tetraborate and the like have been often used for a substrate of a surface acoustic wave device. However, such conventional substrates are difficult to realize the small SAW velocity, the large electromechanical coupling coefficient xcexa2 and the small temperature coefficient of frequency at the same time.
Meanwhile, as means for achieving a filter with a low loss, the use of a unidirectional transducer has been proposed. Transversal surface acoustic wave filters in general are bidirectional and therefore they theoretically induce an insertion loss of at least 6 dB. On the other hand, unidirectional transducers have an electrode structure that counteracts the backward wave and generates only the forward wave by adjusting the phases of the excitation wave and the reflection wave. Therefore, it is theoretically possible to realize an insertion loss of a few dB or less.
Unidirectional transducers are roughly classified into multiple-phase unidirectional transducers and single-phase unidirectional transducers. Furthermore, the single-phase unidirectional transducers include those realizing unidirectionality by installing reflectors in an asymmetrical manner, and natural single-phase unidirectional transducers utilizing the anisotropy of the substrate as described, for example, in Published Unexamined Japanese Patent Application Sho 61-136313.
The single-phase unidirectional transducers realizing unidirectionality by asymmetrically installing reflectors require complex electrode structures and, particularly when applied to a high frequency region, the size of the electrodes is reduced, which brings about difficulty in the production thereof.
On the other hand, the natural single-phase unidirectional transducers utilize the anisotropy of the piezoelectric substrate used, and it is therefore possible to use ordinary interdigitated electrodes. This is advantageous in application to a high frequency region.
However, conventionally known substrates having unidirectional characteristics have such problems that the SAW velocity is large, the electromechanical coupling coefficient xcexa2 is small, the temperature coefficient of frequency is large, and the like.
An object of the invention is to provide a surface acoustic wave device that can achieve a wider band and a small size and can achieve a low loss by utilizing the natural single-phase unidirectional transducer characteristics.
The surface acoustic wave device of the invention comprises: a substrate composed of a single crystal belonging to the point group 32, having the Ca3Ga2Ge4O14 type crystalline structure, including La, Ta, Ga and O as main components and being represented by a chemical formula La3Ta0.5Ga5.5O14; and an electrode formed on the substrate and having a structure that exhibits natural single-phase unidirectional transducer characteristics in cooperation with anisotropy of the substrate.
According to the surface acoustic wave device of the invention, using the substrate and the electrode mentioned above, it is possible to realize a surface acoustic wave device having a large electromechanical coupling coefficient that is advantageous for achieving a wider band, a small SAW velocity that is advantageous for reduction in size, and also having natural single-phase unidirectional transducer characteristics.
In the surface acoustic wave device of the invention, when the cut angle of the substrate out of the single crystal and a propagation direction of a surface acoustic wave are represented by Eulerian angles(xcfx86, xcex8, "psgr"), xcfx86, xcex8 and "psgr" may be in a region where xcfx86 is from xe2x88x925xc2x0 to 5xc2x0, xcex8 is from 135xc2x0 to 155xc2x0, and "psgr" is from 15xc2x0 to 40xc2x0.
In the surface acoustic wave device of the invention, when a cut angle of the substrate out of the single crystal and a propagation direction of a surface acoustic wave are represented by Eulerian angles (xcfx86, xcex8, "psgr"), xcfx86, xcex8 and "psgr" may be in a region where xcfx86 is from 10xc2x0 to 20xc2x0, xcex8 is from 140xc2x0 to 157xc2x0, and "psgr" is from 30xc2x0 to 60xc2x0.
Other objects, features and advantages of the invention will be sufficiently apparent from the following description.